Tricky: By default, Linux seems to mount an NFS root using NFS-V2, even with recent Kernels. I could not find any Documentation about this. Also, in recent editions of NFS-Utils, NFS V2 is disabled by default. Result: Protocol is not supported. No such error messages are displayed anywhere - not on Host NFS logs and not in DMESG of guest Kernel.

Background: In an older installation of nfs-utils, the same VM would come up. After emerge world, nfs-utils got upgraded too and the VM broke. Since all other clients I have natively support all kinds of NFS versions, there seemed to be no error on NFS. And actually, there is none. The way I figured it out was to manually mount a NFS share from the client with the same options my guest kernel used while booting (these are displayed). So, forcing a NFS V2 mount on a client would fail.